In digital reproduction of documents such as in the digital copier environment, a document is first optically scanned and converted to a gray scale image. In the case of color reproduction, the document may be converted to a gray scale image of several separations, such as the R, G and B separations.
In order to produce a hard copy of the scanned and digitized image, the image has to be further processed according to the requirements of the marking engine. For example, if the marking engine is capable of bi-level printing, then the image has to be rendered into a 1-bit bit map for printing. To preserve the appearance of a gray scale image in a binary output, often some digital halftoning process is used in which the multi-bit input image is screened with a periodic array. However, if the original image itself contains halftone screen, objectionable moire patterns may occur due to the interference between the original and the new screens. Also, while dot screen halftoning may be good for rendering continuous tone originals, it may degrade the quality of text and line drawings. Often a document contains different types of images.
In order to achieve optimal image quality in document reproduction, a system capable of automatically identifying different types of images on or within a scanned original image is needed. For example, if an image part is identified as halftone, then some kind of low-pass filtering may be applied prior to halftone screening so the gray scale appearance can be preserved without introducing moire patterns. For text area, some sharpness enhancement filter could be applied and other rendering techniques such as thresholding or error diffusion could be used.
In classifying images or portions of images for the most suitable processing for subsequent printing or other purposes, one specific source of error is the misclassification of areas including closely, regularly spaced lines as a dot halftone screen. This problem is especially acute if the spacing and frequency of the lines in the original image are comparable to the frequency and spacing of dots in a halftone screen to which the algorithm is sensitive. If such a “lined” area is incorrectly characterized as an area of a halftone screen, inappropriate image-processing algorithms may be applied to the area, yielding undesirable image artifacts in, for example, a digital copy of the image. It is therefore desirable to avoid mischaracterization of lined areas of an original image as areas of halftone screen.